Abstract: A method of determining the total playing time T.sub.G and the residual playing time T.sub.R of a magnetic tape (4) stored on two reels (2, 3) in a cassette (1) in which in a brief winding operation of the magnetic tape (4), the numbers of tacho pulses (i.sub.1, i.sub.2) at the take-up reel (3) and the supply reel (2) are determined. Moreover, by extracting the magnetic tape (4) from the cassette (1) over a given length (L.sub.H), the numbers of tacho pulses (j.sub.1, j.sub.2) at the take-up reel (3) and the supply reel (2) are determined. From the values thus determined for these pulses (i.sub.1, i.sub.2, j.sub.1, j.sub.2) for a given thickness d and speed v of the magnetic tape (4), a given reel-hub diameter K of the take-up reel and the supply reel (2, 3) and a given number u of pulses per reel revolution, it is possible to calculate the total playing time T.sub.G in accordance with the relationship: ##EQU1## and the residual playing time T.sub.